The present invention relates to continuously variable belt-drive transmission for a motor vehicle and more particularly to a belt of the transmission in which the compliance at a tight side of the belt is improved.
A known continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions.
FIGS. 2a and 2b show a belt device of the conventional continuously variable belt-drive transmission for a motor vehicle.
The belt-device 1 has an input shaft 2 and an output shaft 4 provided in parallel with the input shaft 2. A drive pulley 3 and a driven pulley 5 are mounted on shafts 2 and 4 respectively. A fixed conical disc 3a of the drive pulley 3 is integral with input shaft 2 and an axially movable conical disc 3b is axially slidably mounted on the input shaft 2. A conical face of the fixed conical disc 3a confronts a conical face of the movable conical disc 3b thereby forming a groove 24a therebetween.
A fixed conical disc 5a of the driven pulley 5 is formed on the output shaft 4 opposite a movable conical disc 5b. Conical faces of the respective discs 5a and 5b form a groove 24b. A belt 6 engages the drive pulley 3 and the driven pulley 5.
The belt 6 comprises a plurality of metal elements 10 adjacently arranged in the longitudinal direction of the belt. Each element has a body portion 12, a head portion 14, a pillar portion 13 at the center and a pair of horizontal slits at both sides thereof between the body portion 12 and head portion 14. A pair of seamless laminated endless metal carriers 30 are inserted in the slits.
Each element 10 has a projection 15 on its one side and a dimple 16 on the other side. The projection 15 of one element 10 engages with the dimple 16 of the adjacent element with a small gap therebetween and all elements are arranged side by side. The belt 6 is thus assembled.
As the movable conical discs 3b and 5b are axially moved along the shafts 2 and 4 for changing the transmission ratio, center lines a and b of the grooves 24a and 24b axially displace, respectively. However, the displacing distance of the line a is not equal to that of the line b. Accordingly, both the lines are inevitably deflected from each other, and the offset .alpha. (the distance between the lines a and b) changes with the change of the transmission ratio. FIG. 2c shows the variation of the offset .alpha. in accordance with the transmission ratio.
In such a device, a tight side 6a of the belt, which is a part of the belt leaving the groove 24a of the drive pulley 3, moves with a deviation from the line a, because of the misalignment of the lines a and b as shown in FIG. 2a. The deviation of the belt is caused by lateral deflection of each element. In addition, many of these elements 10 are arranged at the tight side 6a and they are pressed against each other in the longitudinal direction of the belt, so that the lateral deflections of the elements add up, thereby increasing the deviation of the belt. Further, the force exerted on the element causes the bending and/or tilting of the element which will enhance the deflection. Excessive lateral deflection of the element and the deviation of the belt cause an edge of carrier 30 of the belt 6 to rub against the pillar portions 13 of the elements 10 or an edge of the element entering the groove 24b of driven pulley 5 to strike the conical face of the driven pulley. Additionally, running of the belt 6 becomes unstable when entering the pulleys. As a result, the belt easily wears out and the conical surfaces become rough.
In order to eliminate these drawbacks, a transmission where the absolute value of the offset is decreased by initially offsetting the pulleys is commonly employed. In another transmission, the pulleys are disposed so as to render the offset zero at a small transmission ratio which is the most common driving condition. U.S. Pat. No. 4,596,536 discloses a continuously variable transmission where the drive pulley is so disposed that the offset is zero when the transmission ratio is one, thereby decreasing the amount of rubbing of the carrier against the elements.
However, in such a transmission, the centers of the grooves are initially offset so that amount of the overall offset increases and the loss of the transmission power at the tight side is increased because of the deviation of the belt. In addition, noise caused by a shock which occurs when each element enters the driven pulley increases.